Learn Hardcore Russian: Мой дедушка был для меня авторитетом.

In Russian, when you say that the subject was / became / is something like a profession, role, or status, the noun after быть (to be) is usually in the instrumental:

Мой дедушка был авторитетом. – My grandfather was an authority (figure).
Она была учителем. – She was a teacher.
Он стал врачом. – He became a doctor.

So авторитет → авторитетом (masculine singular instrumental).
The form авторитет (nominative) would be ungrammatical in this sentence.

Why do we say для меня and not just мне?

Для меня literally means “for me” (from my point of view, in relation to me).

для always requires the genitive case, so we use меня (genitive of я), not мне (dative).

Semantically, для меня emphasizes in my eyes / in my view:

Мой дедушка был для меня авторитетом.
My grandfather was an authority figure *for me (in my eyes).*

If you used мне, you’d need a completely different construction (for example, Он был мне как отец – “He was like a father to me”), but you cannot just replace для меня with мне here.

What is the function of для in this sentence?

The preposition для means “for” in the sense of “for someone, from someone’s perspective, intended for someone”.

In Мой дедушка был для меня авторитетом, для меня tells us for whom he was an authority, i.e., from whose perspective.

Для always governs the genitive case, so:

для меня (for me)
для тебя (for you)
для него / для неё (for him / her)

Why is дедушка treated as masculine, even though it ends in -а like a feminine noun?

Дедушка is grammatically masculine, even though it looks like many feminine nouns.

It refers to a male person (grandfather), so it takes masculine pronouns and adjectives:
- мой дедушка (not моя дедушка)
- старый дедушка (not старая дедушка)

The -а / -я ending usually signals feminine, but a few words for male relatives or male persons are exceptions and are masculine:

дедушка – grandfather
мужчина – man
юноша – young man

Could I say Мой дедушка был авторитет для меня without the -ом?

No, that would be incorrect in standard Russian.

After forms of быть (“was / is / will be”) in sentences like this, the noun that names the role/status is normally in the instrumental:

был авторитетом – correct
был авторитет – incorrect here

You can see bare nominative in some other patterns (like short definitions or headlines), but not in this kind of full sentence with был.

Why do we need был here? In present tense, I often see Russian drop есть.

In the present tense, Russian usually omits the verb быть (“to be”):

Мой дедушка — авторитет для меня.
My grandfather is an authority for me.

But in the past tense, you must use a past form of быть:

Мой дедушка был для меня авторитетом. – …was an authority for me.

Without был, the past tense would not be clearly shown; it would sound wrong or incomplete. So:

Present: Мой дедушка — авторитет для меня.
Past: Мой дедушка был для меня авторитетом.

Can I change the word order? For example: Мой дедушка был авторитетом для меня or Для меня мой дедушка был авторитетом?

Yes, both are grammatically correct; the differences are about emphasis:

Мой дедушка был для меня авторитетом.
– Neutral, very natural. Light emphasis on “for me”.
Мой дедушка был авторитетом для меня.
– Very similar meaning; slightly more focus on the role авторитетом first, then you add для меня as clarification.
Для меня мой дедушка был авторитетом.
– Stronger emphasis on для меня (“for me my grandfather was an authority”, maybe implying that for others he might not have been).

All three are fine; choice depends on what you want to stress.

What exactly does авторитет mean here? Is it like “authority”, “expert”, or “role model”?

In this context, авторитет means roughly:

“authority figure”,
“someone whose opinion you deeply respect and follow”,
a kind of moral or personal authority, often close to “role model”.

So Мой дедушка был для меня авторитетом is like:

My grandfather was an authority figure for me.
My grandfather was someone whose opinion I greatly respected.

Note: In some other contexts, авторитет can mean:

a respected influential person in a field, or
colloquially, a criminal boss (in criminal slang).

Here, though, it’s clearly positive: a respected, influential figure in your life.

Why is it меня and not я or мне after для?

Because для always requires the genitive case.

Personal pronouns in the genitive are:

я → меня
ты → тебя
он → его
она → её, etc.

So with для:

для меня – for me
для тебя – for you
для него – for him

Я is nominative (subject form), мне is dative. Neither can follow для.

Could I say Мой дедушка был моим авторитетом instead of …был для меня авторитетом? Any difference?

Yes, Мой дедушка был моим авторитетом is grammatically correct and natural.

Nuance:

был для меня авторитетом
– emphasizes “for me / in my eyes” he was an authority.
был моим авторитетом
– emphasizes possessive (“he was my authority”), almost like he was the key authority figure that I had.

In many contexts they’re close in meaning and interchangeable, but для меня highlights your perspective, while моим highlights belonging / personal attachment a bit more.

How do you pronounce this sentence, and where is the stress in each word?

Pronunciation with stress marked in capitals (approximate English-like transcription):

Мой – MOY (like “moy”)
дЕдушка – DYED-oo-shka; stress on ДЕ
бЫл – byl (short, like “bɨl”; ы is a central vowel)
для – dlya (consonant cluster “dly-”)
менЯ – me-NYA; stress on -ня
авторитЕтом – av-ta-ri-TYE-tam; stress on -те-

So spoken slowly:
МОЙ ДЕдушка был для менЯ авторитЕтом.

Could I say something like становился для меня авторитетом instead of был? What’s the difference?

Yes, you can, but the meaning changes slightly.

был для меня авторитетом – a state in the past:
He was an authority figure for me (already established).
становился для меня авторитетом – a process of becoming:
He was becoming an authority figure for me / He gradually became an authority for me.

Use становился if you want to stress the development over time, not just the final state.